Takahm, charlieh, CTL, there is nothing magical nor (chas v’shalom) was there anything arbitrary about the change in status of peanut oil on Pesach.
I’m not going to elaborate as I am no way learned enough to offer you an appropriate dissection of the relevant responsa from the Rishonim to very late Achronim to the growth of Yiddishkeit in the Americas. But I’m sure if you do your own research you’ll find it’s not at all as simple as it seems.